[Python-ideas] Format mini-language for lakh and crore

[Eric V. Smith]

On 1/29/2018 2:13 AM, Nick Coghlan wrote:
> On 29 January 2018 at 11:48, Nathaniel Smith <njs at pobox.com> wrote:
>> On Sun, Jan 28, 2018 at 5:31 PM, David Mertz <mertz at gnosis.cx> wrote:
>>> I actually didn't know about `locale.format("%d", 10e9, grouping=True)`.
>>> But it's still much less general than having the option in the
>>> f-string/.format() mini-language.  This is really about the formatted
>>> string, not necessarily about the locale.  So, e.g. I'd like to be able to
>>> write:
>>>
>>>>>> print(f"In European format x is {x:,.2f}, in Indian format it is
>>>>>> {x:`.2f}")
>>>
>>> I don't want the format necessarily to be some pseudo-global setting, even
>>> if it can get stored in thread-locals.  That said, having a locale-aware
>>> symbol for delimiting numbers in the format mini-language would also not be
>>> a bad thing.
>>
>> I don't understand the format mini-language well enough to know what
>> would fit in, but maybe some way to (a) request localified formatting,
> 
> Given the example, I think a more useful approach would be to allow an
> optional digit grouping specifier after the comma separator, and allow
> the separator to be repeated to indicate non-uniform groupings in the
> lower order digits.
> 
> If we did that, then David's example could become:
> 
>      >>> print(f"In European format x is {x:,.2f}, in Indian format it
> is {x:,2,3.2f}")

This just seems too complicated to me, and is overgeneralizing. How many 
of these different formats would ever really be used? Can you really 
expect someone to remember what that means by looking at it?

If you are going to generalize it, at least go all the way and support 
the struct lconv "CHAR_MAX" behavior, too.

However, I suggest just pick another character to use instead of ",", 
and have it mean the 2,3 format. With no evidence (and willing to be 
wrong), it seems like it's the next-most needed variety of this. Maybe 
use ";"?

Eric

> 
> The core elements of interpreting that would then be:
> 
> - digit group size specifiers are permited for both "," (decimal
> display only) and "_" (all display bases)
> - if no digit group size specifier is given, it defaults to 3 for
> decimal and 4 for binary, octal, and hexadecimal
> - if multiple digit group specifiers are given, then the last one
> given is applied starting from the least significant integer digit
> 
> so "{x:,2,3.2f}" means:
> 
> - an arbitrary number of leading 2-digit groups
> - 1 group of 3 digits
> - 2 decimal places
> 
> It would then be reasonably straightforward to use this as a lower
> level primitive to implement locale dependent formatting, as follows:
> 
>      - format in English using the locale's grouping rules [1] (either
> LC_NUMERIC.grouping or LC_MONETARY.mon_grouping, as appropriate)
>      - use str.translate() [2] to replace "," and "." with the locale's
> thousands_sep & decimal_point or mon_thousands_sep & mon_decimal_point
> 
> [1] https://docs.python.org/3/library/locale.html#locale.localeconv
> [2] https://docs.python.org/3/library/stdtypes.html#str.translate
> 
> Cheers,
> Nick.
>